It's actually 100% guaranted, but I see how some people might find it confusing. You need to reach climax during a fertile day, there are many ways to do it: maximize the SIZE stat on the shop, use special candy before entering the level, match son/mother during the level, change positions a couple time during the level, keep having sex during the yellow sign phase (risky, but drastically increase the heat bar)... After you manage to achieve a climax, there is a insemination minigame, to beat it you need to maximize your STAMINA on the shop.
Basically: maximize SIZE and STAMINA, then play the lemonade sale on a fertile day. It only costs 200 to maximize both.
Thanx a lot for sharing your thoughts! :)